Cemented carbides (WC-base sintered alloys) are known as alloys for cutting tool materials or wear-resistant tool materials. However, the cutting tools of the cemented carbides are easily worn at rake faces by cutting steel. In order to solve this problem, cermet alloys have been developed. For example, TiC-base cermets containing TiC as main components have been developed, however, these materials are insufficient in toughness, and therefore TiCN-base cermets further containing TiN are widely used.
It is known that modifying the hard particle in the TiCN-base cermets into a core-containing structure particle having a core and a shell can improve the hardness and the toughness of the TiCN-base cermets. The hard particle has most influence on the mechanical property of the TiCN-base cermets (see Japanese Unexamined Patent Application Publication No. 2-254131, and Japanese Unexamined Patent Application Publication No. 10-287946 etc.). The contents of these publications are incorporated herein by reference in their entirety.
Japanese Unexamined Patent Application Publication No. 3-170637 discloses a cermet which includes hard particles having cores and shells composed of carbonitrides of metals (hard metals) belonging to group IVa, Va, or VIa of the periodic table. The hard particles include plural kinds of core-containing structure particles which have cores and/or shells composed of plural kinds of the hard metals. The cermet can improve the defect resistance (toughness) without a decrease in wear resistance (cutting resistance). The contents of this publication are incorporated herein by reference in their entirety.
Japanese Unexamined Patent Application Publication No. 11-229068 discloses that sintering properties can be improved by dispersing superfine alloy particles composed of a binding metal made of Co and/or Ni in hard particles having a core-containing structure, and thereby even cermets having a small binding phase can be densified. The contents of this publication are incorporated herein by reference in their entirety.
However, hard particles having a core-containing structure, such as those disclosed in Japanese Unexamined Patent Application Publications No. 2-254131 and No. 10-287946, have limitation in improvement of mechanical properties and cutting performance. In particular, TiCN-base cermets whose thermal shock resistance and chipping resistance are equal to those of a WC-base sintered alloy provided with a hard coating film on the surface have been desired.
In the particles having a plurality of core-containing structures whose cores and shells have different compositions each other, as those disclosed in Japanese Unexamined Patent Application Publication No. 3-170637, since the hard particles consist of only hard metals such as carbonitride, the thermal conductivity of a cermet is low. Therefore, generated heat in a cutting edge due to cutting cannot be efficiently dissipated. As a result, disadvantageously, the temperature of the cutting edge increases and thermal shock resistance and chipping resistance decreases.
Furthermore, as in Japanese Unexamined Patent Application Publication No. 3-170637, dispersing superfine alloy particles made of a binding metal in hard particles improves the sintering properties of the cermet. However, a binding metal having a low hardness is present in the form of particles. Besides, the binding phase is originally in a low proportion and has a low binding force. Therefore, the strength of the sinter decreases and the binding metal particles may cause breakage or chipping.